RNA tumor viruses replicate through a DNA intermediate (provirus), which is eventually incorporated into the host chromosomal DNA. Synthesis of several intermediate forms in avian cells infected by avian sarcoma virus appears to precede the formation of a covalently closed duplex circular molecule. The characteristics of these forms, with respect to the size and structure, will be studied. The technique of nucleic acid hybridization will be used to detect viral DNA intermediates. Size, structure and composition of various forms will be determined in velocity and equilibrium density gradients. The function of the isolated viral genome, regulation of the expression of viral RNA transcription will also be explored.